1. Field of the Invention
The present invention relates to a process for the production of light-colored uretdione polyisocyanates and to their use as a starting component for polyurethane plastics, particularly as an isocyanate component for the production of uretdione powder coating hardeners.
2. Description of the Prior Art
The production of aliphatic or cycloaliphatic polyisocyanates having uretdione groups by the catalytic dimerization and optionally trimerization (generic term: oligomerization) of monomeric aliphatic or cycloaliphatic diisocyanates is known and described, e.g., in J. Prakt. Chem. 336 (1994) 185-200. In accordance with the present invention the term "oligomerization" refers to the dimerization and optional trimerization of isocyanates.
The uretdione polyisocyanates obtained by these known processes exhibit a marked inherent color depending upon the type of catalyst used. This color severely limits their potential for use as a starting component for the production of high-grade surface coating resins. Numerous attempts have been made to improve the color quality of uretdione polyisocyanates.
It is known from EP-A 337 116 that carbon dioxide dissolved in the starting monomer has a considerable influence on the color of the products during the phosphine-catalyzed dimerization of aliphatic diisocyanates. Only after removing the dissolved carbon dioxide (e.g., to a residual content of less than 20 ppm) can light-colored uretdione polyisocyanates be produced from 1,6-diisocyanatohexane (HDI).
EP-A 377 177 describes the production of practically colorless HDI uretdiones by tributylphosphine catalysis in the presence of alcoholic co-catalysts and subsequent oxidative brightening of the low-monomer resins obtained after thin-layer distillation with the aid of organic peroxides. However, the handling of suitable peroxides is not without problems, especially on an industrial scale, and in some cases requires considerable safety precautions.
It is proposed in EP-A 569 804 to use (atmospheric) oxygen for the oxidative brightening of aliphatic or cycloaliphatic uretdione polyisocyanates. However, the desired effect occurs only above 80.degree. C., i.e., at temperatures which are sufficient to thermally decompose the uretdione groups. For this reason, products having a low monomer content can only be obtained with difficulty by this process.
EP-A 735 027 describes a variation of the process disclosed in EP-A 317 744 for the production of (cyclo)aliphatic uretdiones by catalysis with 4-dialkylaminopyridines, such as 4-dimethylaminopyridine (DMAP). In this process the dimerization is carried out in the presence of aromatic or araliphatic phosphines or alkyl phosphites with up to 8 carbon atoms in the alkyl radicals. This modified process does provide products with improved color quality, but this is often not sufficient for special applications, for example for the production of uretdione hardeners for clear polyurethane powder coatings. Also, the phosphorus compounds described in EP-A 735 027 are relatively low molecular weight, in some cases foul-smelling, toxic substances, which makes them difficult to handle.
An object of the present invention is to provide a new process for the production of uretdione polyisocyanates which results in light-colored products and does not have the disadvantages previously set forth.
This object may be achieved with the process according to the invention, which is based on the surprising observation that the dimerization of aliphatic or cycloaliphatic diisocyanates with known catalysts results in uretdione polyisocyanates with considerably lower color values than comparable prior art processes if small quantities of special trisubstituted phosphites are added to the reaction mixture.